1. Field of the Invention
The present invention relates to an implantable medical device for continuous, minimally invasive monitoring of a chemical analyte of interest using an expandable, biocompatible material incorporated into an electronic circuit component that is interrogated transdermally.
2. Description of Related Art
Various implantable medical monitoring devices have been developed to measure internal physiological conditions of a patient. For example, an implantable medical sensor that determines the oxygen content of blood using a light-emitting diode and a phototransistor is described in U.S. Pat. No. 4,815,469 to Cohen et al. U.S. Pat. No. 5,709,225 to Budgifvars et al. describes a medical implant with a capacitive sensor, which is coated with a magnetically sensitive material that causes capacitance changes in the presence of a magnetic field.
Some medical devices use sensors coupled with frequency tuned L-C circuits, where the sensor mechanically translates the changes in the physiological condition to the inductor or capacitor of the tuned L-C circuit. An external transmitter detects the resulting changes in resonant frequency of the circuit. For example, U.S. Pat. No. 5,704,352 to Tremblay et al. describes an implantable passive biosensor for monitoring physiological conditions and converting the signals to digital format. In particular, the sensors are pressure transducers that detect the pressure of cerebrospinal fluid in the cavities of a patient""s brain, which is useful for monitoring the operation of a cerebrospinal fluid shunt for treating hydrocephalus. U.S. Pat. No. 4,114,606 to Seylar describes an implantable device useful for monitoring intractanial pressure for the treatment of hypertension. The resonant frequency of the passive L-C circuit implanted in the cranium varies with changes in intracranial pressure. An external monitor interrogates and detects the frequency changes in the pressure transducer. Neither of these patents describes specific mechanisms or devices for transducing other physiological conditions.
The application of a transducer in an implantable medical device that reliably monitors changes in specific chemical analytes, such as blood glucose, would be advantageous. Blood glucose levels are of particular concern because diabetes is a chronic illness that affects more than 110 million people worldwide. Conventional therapy for the most severe form of diabetes, insulin-dependent diabetes mellitus (Type I), is to administer one or more injections per day of various forms of insulin, while monitoring blood glucose levels two or three times daily with commercial glucometers that require the withdrawal of blood samples. In practice, near normal blood sugar levels are difficult to maintain with this type of therapy due to the enormous inconvenience and day-to-day burden of conventional home-monitoring techniques. The resulting large fluctuations in blood glucose levels may be responsible for a number of serious secondary ailments commonly associated with diabetes, including stroke, liver and kidney damage, and loss of eyesight.
The present invention addresses the need for a convenient, minimally invasive medical sensor that provides continuous measurements of an analyte of interest using an expandable, biocompatible material incorporated into an electronic circuit element. More specifically, this invention will help diabetic patients in monitoring blood glucose levels and achieving tighter blood glucose control without requiring blood samples to be drawn.
The present invention provides an implantable sensor system for monitoring the concentration of a chemical analyte of interest. The invention is used for medical applications, such as implanted sensor packages for long-term monitoring of physiological blood or tissue analytes, like glucose for control of diabetes. The analyte concentration is transduced by a circuit, the characteristics (e.g., resonant frequency) of which are set by at least one circuit component (e.g., capacitance, inductance, resistance) whose value can be varied by the interaction between an analyte-sensitive material and the analyte. For example, changing the distance between the plates with a glucose-swellable polymer can vary the capacitance of a parallel-plate capacitor. As the electrical characteristics of the circuit vary in response to changes in the concentration of the analyte, an external interrogator transmits a signal transdermally to the transducer, and the concentration of the analyte is determined from the response of the transducer to that signal.
It is an object of the present invention to provide an implantable sensor system to monitor one or more chemical analytes of interest, including ionic species and molecular species. It is also an object to provide an implantable transducer having a circuit that requires no internal source of power, and which incorporates an expandable material that changes its dimensions in the presence of the analyte to influence the properties of the transducer circuit. Another object of the invention is to provide a sensor system interrogated transdermally by an external device to measure the characteristics of the circuit as the concentration of analyte changes. It is further an object of the invention to provide an implantable sensor system for monitoring the blood glucose levels in diabetic patients. Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.